Beam splitter/combiner for optical metrology tool

ABSTRACT

A combiner for optical beams includes a substrate overlaid by a multi-layer dielectric film stack. The substrate is a clear material and the dielectric film stack is a series of alternating layer of high and low refractive index. This gives the combiner relatively high reflectivity across UV wavelengths and relatively high transmissivity in the visible and longer wavelengths and allows visible light to pass through the combiner while UV light is reflected. At the same time dielectric film stack has minimal absorption and scatter. This means that the intensity of visible light maintains at least 90% of its intensity as it passes through combiner and UV light retains at least 90% of its intensity as it is reflected by combiner.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.10/229,630, filed Aug. 28, 2002. This application claims priority toU.S. Provisional Patent Application Ser. No. 60/363,110, filed Mar. 11,2002, which is incorporated herein by reference.

TECHNICAL FIELD

The subject invention relates to a device for combining two beams oflight with minimal loss to create a single output beam.

BACKGROUND OF THE INVENTION

Over the past several years, there has been considerable interest inusing optical methods to perform non-destructive inspection and analysisof semi-conductor wafers. The type of inspection is commonly referred toas optical metrology and is typically performed using a range of relatedtechniques including ellipsometry and reflectometry. At the heart ofthese techniques is the notion that a subject may be examined byanalyzing the reflection of a probe beam that is directed at thesubject. For the specific case of ellipsometry, changes in thepolarization state of the probe beam are analyzed. Reflectometry issimilar, except that changes in magnitude are analyzed. Ellipsometry andreflectometry are effective methods for measuring a wide range ofattributes including information about thickness, crystallinity,composition and refractive index. The structural details ofellipsometers are described more fully in U.S. Pat. Nos. 5,910,842 and5,798,837 both of which are incorporated in this document by reference.

Scatterometry is a related technique that measures the diffraction(optical scattering) that results when a probe beam is directed at asubject. Scatterometry is an effective method for measuring the criticaldimension (CD) of structural features (such as the lines and otherstructures included in integrated circuits). Scatterometry can be usedto analyze two periodic two-dimensional structures (e.g., line gratings)as well as periodic three-dimensional structures (e.g., patterns of viasor mesas in semiconductors). Scatterometry can also be used to performoverlay registration measurements. Overlay measurements attempt tomeasure the degree of alignment between successive lithographic masklayers.

Most metrology techniques (including those just described) may beperformed using monochromatic or polychromatic light. In the case wherepolychromatic light is used, the interaction between the probe beam andthe subject is analyzed as a function of wavelength. In many cases, thisincreases the accuracy of the analysis. As shown in FIG. 1, arepresentative implementation of an ellipsometer or reflectometerconfigured to perform this type of polychromatic analysis includes abroadband light source. The light source creates a polychromatic probebeam that is focused by one or more lenses on a subject. The subjectreflects the probe beam. The reflected probe beam passes through anotherseries of one or more lenses to a detector. A processor analyzes themeasurements made by the detector.

The broadband light source is a combination of two different sources: avisible light source and a UV source. The visible light source istypically a tungsten lamp and the UV source is typically a deuteriumlamp. The outputs of the two lamps are combined using a beam combiner.Prior art beam combiners are usually formed by depositing a very thinpartially transparent metal film, such as aluminum on a substrate. Thesurface of the film is coated with a protective layer of silicon dioxideor magnesium fluoride. A notable example of a UV to visible beamcombiner is a 50/50 beam splitter. The output of the beam combiner isthe probe beam produced by the broadband light source. The combinationof the two separate lamps increases the spectrum of the probe beambeyond what would be practical using a single source.

Unfortunately, the use of prior art beam combiners has known drawbacks.As shown in FIG. 2, a portion of the beam produced by the visible lightsource is lost because it is reflected instead of being transmitted bythe combiner. The output of the UV light source suffers the oppositefate. A portion of that beam is lost because it is transmitted insteadof being reflected by the combiner. An additional portion of each beamis lost through absorption and scatter during interaction with thecombiner. The overall result is that the intensity of the combined probebeam is significantly reduced when compared to the sum of the outputsproduced by the visible and UV light sources. For a 50/50 beam combiner,the intensity of the combined probe beam can be 30% of the sum of theoutputs produced by the visible and UV light sources.

For these reasons and others, a need exists for improved devices forcombining optical beams. This need is especially important for metrologytools, which require the combination of multiple illumination sources tocreate wide spectrum polychromatic probe beams.

SUMMARY OF THE INVENTION

The present invention provides a combiner for optical beams. The beamcombiner includes a substrate overlaid by a multi-layer dielectric filmstack. The substrate is formed from a transparent material such as fusedsilica. The film stack is designed to provide relatively highreflectivity across the UV wavelengths and relatively hightransmissivity in the visible and longer wavelengths. During normaloperation, the combiner is positioned at the perpendicular intersectionof two beams: a visible light beam and a UV light beam. In thisposition, the transmissivity of the combiner allows the visible lightbeam to pass unimpeded. The reflectivity of the combiner redirects theUV light beam to coaxially follow the visible light beam. The result isa single output beam that combines the UV and visible light beams. Thefilm stack has minimal absorption and scatter so more than 90% of thevisible and UV light beams are transferred to the output beam.

To create the required combination of transmissivity (for visible light)and reflectivity (for UV light) the dielectric film stack is configuredto include alternating layers of high and low refractive indexmaterials. For a representative embodiment, two series of layers areused. The first series includes a total of twenty-eight layers. The lowrefractive index layers in the first series are formed using silicondioxide (SiO₂). Scandium oxide (Sc₂O₃) is used for the high refractiveindex layers. The second series in the first series includes a total offourteen layers. The low refractive index layers in the second seriesare formed using magnesium fluoride (MgF₂). Aluminum oxide (Al₂O₃) isused for the high refractive index layers.

The combiner may also act as a beam splitter. When positioned in thepath of a beam including both visible and UV light, the combinerproduces two output beams. The first output beam includes the visiblecomponent of the input beam and the second includes the UV component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an optical metrology system shown as arepresentative use for a beam combiner.

FIG. 2 is a diagram showing operation of a prior art beam combiner.

FIG. 3 is a diagram of a combiner for optical beams as provided by thepresent invention.

FIG. 4 is a plot of reflectivity and transmissivity as a function ofwavelength for an embodiment of the combiner for optical beams providedby the present invention.

FIG. 5 is a diagram showing the combiner of the present invention beingused to combine optical beams.

FIG. 6 is a diagram showing the combiner of the present invention beingused to split an optical beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a combiner for optical beams. As shown inFIG. 3, a representative implementation of the combiner 300 includes asubstrate 302 overlaid by a multi-layer dielectric film stack 304.Substrate 302 may be fabricated using a range of transparent materials.For the particular embodiment being described fused silica is used.

The film stack 304 is designed to provide relatively high reflectivityacross UV wavelengths and relatively high transmissivity in the visibleand longer wavelengths. As shown in FIG. 3, this allows visible light topass through combiner 300 while UV light is reflected. At the same timedielectric film stack 304 has minimal absorption and scatter. This meansthat the intensity of visible light maintains at least 90% of itsintensity as it passes through combiner 300. Similarly, UV light retainsat least 90% of its intensity as it is reflected by combiner 300.

To create the required combination of transmissivity (for visible light)and reflectivity (for UV light) dielectric film stack 304 is configuredto include alternating layers of high and low refractive indexmaterials. For a representative embodiment, a total of twenty-eightlayers are used (of which only four are shown in FIG. 3). Differentembodiments may use more or less layers. The low refractive index layersare formed using silicon dioxide (SiO₂). Scandium oxide (Sc₂O₃) is usedfor the high refractive index layers.

Each of the layers within dielectric film stack 304 is typically formedwith an approximate quarter-wave optical thickness. To reducereflections in the visible and IR bands, it is possible to modify thethickness of individual layers within dielectric film stack 304. Inpractice, it has been found that one or more of the outer layers on eachside of dielectric film stack 304 should be optimized in this fashion.

In practice, the combination of layers just described provides goodreflectivity in the ultraviolet range above 215 nm. To increase thisrange, additional layers may be added at the top (non-substrate side) ofdielectric film stack 304. For a representative embodiment, fourteenadditional layers are added using magnesium fluoride (MgF₂) for the lowrefractive index layers and aluminum oxide (Al₂O₃) for the highrefractive index layers. This provides good reflectivity in theultraviolet range above 193 nm.

Other materials may be used for the additional layers includingNeodymium Fluoride (NdF₃), Gadolinium Fluoride (GdF₃), LanthanumFluoride (Laf₃), Aluminum Oxide (Al₂O₃), Praseodymium Fluoride (PrF₃)and Thorium Fluoride (ThF₄) for the high refractive index layers andAluminum Fluoride (AlF₃), Magnesium Fluoride (MgF₂), Silicon Dioxide(SiO₂), Lithium Fluoride (LiF), and Cryolite (Na₃AlF₆) for the lowrefractive index layers.

FIG. 4 shows a plot of reflectivity and transmissivity as a function ofwavelength for the described embodiment of twenty-eight initial layerswith fourteen additional layers. As shown in FIG. 4, reflectivity ishigh (and transmissivity is low) within the UV range of 180 to 350 nm.Within the visible range of 400 to 800 nm, reflectivity is low (andtransmissivity is high).

As shown in FIG. 5, the combiner 300 typically is positioned in the pathof two beams: a visible light beam and a UV light beam. For mostapplications, these beams are mutually perpendicular and coplanar andthe combiner 300 is positioned at their intersection. In this position,the transmissivity of the combiner 300 allows the visible light beam topass unimpeded. The reflectivity of the combiner 300 redirects the UVlight beam to coaxially follow the visible light beam. The result is asingle output beam that combines the UV and visible light beams. Theminimal absorption and scatter of the dielectric film stack 304 meansthat more than 90% of the visible and UV light beams is transferred tothe output beam.

For typical optical metrology applications, the UV light beam of FIG. 5is generated by a deuterium lamp. The visible light beam can be providedby a tungsten, xenon or halogen lamp. In order to create light into theinfrared, a halogen lamp is used. For the specific combination of adeuterium lamp with a halogen lamp there is a known drop in intensityaround the 400 nm region. For cases of this type, the layers withindielectric film stack 304 are reconfigured to reduce transmission andreflection above and below 400 nm in order to smooth out the intensitylevel of the beam across the entire wavelength range.

As shown in FIG. 6, the combiner 300 may also act as a beam splitter.When positioned in the path of a mixed beam including both visible andUV light, the combiner produces two output beams. The first output beamincludes the visible component of the input beam and the second includesthe UV component. In optical metrology tools, the type of configurationis particularly useful for splitting the light that has been reflectedby the subject. Each separate component may then be passed to a separatedetector.

In the preceding discussion, particular attention has been devoted tothe use of combiner 300 within optical metrology tools. There are, ofcourse, a wide range of systems of all types that use opticalcomponents. Combiner 300 is specifically intended to be useful acrossthis range of systems and is not intended to be limited to the field ofoptical metrology.

1. A light source for an optical metrology tool which comprises: a firstsource producing a first light beam including broadband visibleradiation; a second source producing a second light beam includingbroadband ultraviolet radiation; and a beam combiner positioned in thepath of the first and second light beams, the beam combiner including amultilayer dielectric film stack, the multilayer dielectric film stackbeing substantially transmissive to the first light beam so that thefirst light beam is transmitted through the combiner, the multilayerdielectric film stack being substantially reflective to the second lightbeam so that the second light beam is reflected to follow the visiblelight beam.
 2. A light source as recited in claim 1, wherein in thefirst source is tungsten lamp and the second source is a deuterium lamp.3. A light source as recited in claim 1, wherein the first source isxenon lamp and the second source is a deuterium lamp.
 4. A light sourceas recited in claim 3, wherein the multilayer dielectric film stackincludes a first series of alternating layers of high and low refractiveindex materials.
 5. A light source as recited in claim 4, wherein thelayers within the first series of high refractive index materials arecomposed of scandium oxide (Sc₂O₃) and the layers within the firstseries of low refractive index materials are composed of silicon dioxide(SiO₂).
 6. A light source for an optical metrology tool which comprises:a xenon lamp for producing a first broadband light beam; a deuteriumlamp for producing a second broadband light beam; and a beam combinerpositioned in the path of the first and second light beams, the beamcombiner including a multilayer dielectric film stack, the multilayerdielectric film stack being substantially transmissive to first lightbeam so that the first light beam is transmitted through the combiner,the multilayer dielectric film stack being substantially reflective tothe second light beam so that the second light beam is reflected tofollow the first light beam.
 7. A light source as recited in claim 6,wherein the multilayer dielectric film stack includes a first series ofalternating layers of high and low refractive index materials.
 8. Alight source as recited in claim 7, wherein the layers within the firstseries of high refractive index materials are composed of scandium oxide(Sc₂O₃) and the layers within the first series of low refractive indexmaterials are composed of silicon dioxide (SiO₂).
 9. A device foroptically inspecting and evaluating a subject, the device comprising:(a) a first source for producing a first beam including broadbandvisible radiation; (b) a second source for producing a second light beamincluding broadband ultraviolet radiation; (c) a beam combinerpositioned in the path of the first and second light beams, the beamcombiner including a multilayer dielectric film stack, the multilayerdielectric film stack being substantially transmissive to the firstlight beam so that the first light beam is transmitted through thecombiner, the multilayer dielectric film stack being substantiallyreflective to the second light beam so that the second light beam isreflected to follow the first light beam; (d) at least one opticalelement for focusing the combined first and second light beams on thesubject; (e) a detector for measuring the light reflected from thesubject; and (f) a processor for analyzing the measurements made by thedetector.
 10. A device as recited in claim 9, wherein in the firstsource is tungsten lamp and the second source is a deuterium lamp.
 11. Adevice source as recited in claim 9, wherein the first source is xenonlamp and the second source is a deuterium lamp.
 12. A device as recitedin claim 11, wherein the multilayer dielectric film stack includes afirst series of alternating layers of high and low refractive indexmaterials.
 13. A device as recited in claim 12, wherein the layerswithin the first series of high refractive index materials are composedof scandium oxide (Sc₂O₃) and the layers within the first series of lowrefractive index materials are composed of silicon dioxide (SiO₂).
 14. Adevice for optically inspecting and evaluating a subject, the devicecomprising: (a) a xenon lamp for producing a first broadband light beam;(b) a deuterium lamp for producing a second broadband light beam; (c) abeam combiner positioned in the path of the first and second lightbeams, the beam combiner including a multilayer dielectric film stack,the multilayer dielectric film stack being substantially transmissive tothe first light beam so that the first light beam is transmitted throughthe combiner, the multilayer dielectric film stack being substantiallyreflective to the second light beam so that the second light beam isreflected to follow the first light beam; (d) at least one opticalelement for focusing the combined first and second light beams on thesubject; (e) a detector for measuring the light reflected from thesubject; and (f) a processor for analyzing the measurements made by thedetector.
 15. A device as recited in claim 14, wherein the multilayerdielectric film stack includes a first series of alternating layers ofhigh and low refractive index materials.
 16. A device as recited inclaim 15, wherein the layers within the first series of high refractiveindex materials are composed of scandium oxide (Sc₂O₃) and the layerswithin the first series of low refractive index materials are composedof silicon dioxide (SiO₂).